1. Field of the Invention
The present invention relates generally to registration references for the print head of a high resolution laser or ink-jet printer or a plotter, and in particular to method of manufacturing a true-dimension optical encoder strip for a wide-format printer or plotter manufactured using known imagesetting devices.
2. Description of the Prior Art
Printers and plotters utilizing a wide variety of technologies are well known to the art. The term xe2x80x9cprintersxe2x80x9d is used herein generically referring to color and monochromatic (black-and-white) laser printers, inkjet printers, and plotters, unless a particular distinction between these types of devices is specifically called for or noted. In general, many printers and plotters operate using substantially similar or interchangeable technology and components, but are utilized in different applications. Those of skill in the art readily appreciate these distinctions or limitations, and the relative advantages or disadvantages of the corresponding technologies.
Many commercial and personal printers have resolutions of 300 dots per inch (dpi) or greater, with 600 dpi and 1200 dpi becoming standard within recent years. Resolutions of greater than 2000 dpi can be achieved on some high end personal printers, and are conventional for professional printers, typesetting machines, and photoduplicating or photolithography machines.
It is readily appreciated that such printers require great precision and uniformity in the ability to repeatably position the print head. Variations in this precision result in compression or expansion along individual lines of an image, or in elements which lack clarity or definition at the desired dot resolution. Variations between lines will result in abnormally dithered or skewed portions of images, or other irregularities in print quality or clarity. In many graphic images for personal or even professional use, these minor variations will not be readily detectable by normal visual inspection in most applications unless a particular screen pattern or color separation is involved which produces a cascade effect and creates visible distortions throughout larger portions of the total image. By comparison, this lack of precision cannot be tolerated for computer-aided design (CAD) applications. In the case of high resolution or enhanced resolution printers for professional applications, precise print head placement is required to achieve the expected dot resolution of the device over the entire width of the image. Because high resolution images in large formats can be very expensive and slow to produce, plotters are more frequently utilized in applications where a large format image is created (often composed of significant xe2x80x9cwhite spacexe2x80x9d), but exact accuracy is expected in line weights and the spacing between individual lines, the curvature or length of lines, and the density of image elements.
As such, providing an accurate linear reference to uniformly and repeatably determine the registration or placement of a print head is indispensable for printers and plotters. Many such devices rely on encoder strips which ideally have a multiplicity of discrete markings, equally spaced from one another but without corresponding dimensional references such as inches or points relative to the terminal ends of the encoder strip. A sensor such as an optical emitter/detector is mounted on or near the print head or carriage, and produces a digital or analog signal pulse as the sensor passes and detects each marking. A count of the signal pulses is used to calculate the position of the print head relative to one of the terminal ends of the encoder strip, or to the last reference position of the print head.
However, in practice the uniformity or precision in the spacing and weight of markings on an encoder strip is very much less than ideal. This is due primarily to limitations in the fabrication processes which result in inaccurate registration references. FIGS. 1 and 2 illustrate a prior art encoder strip having an irregular spacing defect or xe2x80x9cbandingxe2x80x9d defect. A distance between adjacent registrations marks 16 varies across the length of the encoder strip. As further discussed herein, the xe2x80x9cbandingxe2x80x9d defect may result from limitations of the imagesetter""s control and software systems.
Encoder strips fabricated from a polymer sheet or film such as Mylar(copyright) are also known. The markings on these polymer film encoder strips may be imprinted in a variety of ways, however the ultimate accuracy of the encoder strip is limited by the precision of the imprinting process or apparatus. Very high resolutions for imprinting encoder strips can be achieved using a device such as a laser imagesetter designed for electronic tooling, printed circuit board (PCB) fabrication, and wafer photoetching processes. Such imaging systems may be of either planar and drum design. Planar imaging systems, such as disclosed in U.S. Pat. No. 4,841,656, are types of imaging systems which have a planar surface for receiving a substrate. An optical exposure head is located on a movable gantry apparatus and is rastered above the substrate during exposure. Drum imaging systems, which may be of external or internal drum design, have a cylindrical drum surface portion receiving a substrate. A reflected or directed light beam is advanced across the substrate surface during exposure. Examples of such drum imaging systems are disclosed in U.S. Pat. Nos. 5,841,567 and 5,828,501.
A fundamental flaw has existed in the manufacture of encoder strips used for wide format printers. This deficiency is the result of reliance by those of skill in the art on traditional xe2x80x9clines per inchxe2x80x9d standards for calculating and controlling image resolution. For example, one inch (1xe2x80x3) of encoder strip imprinted for 300 dpi basic (physical) resolution would have an alternating pattern of 150 lines and 150 intervening spaces. However, each line and each space would be one three-hundredths of an inch ({fraction (1/300)}xe2x80x3) in width. Converting this to decimal form, each line (or space) would have a width of 0.00333333 . . . inches, wherein the row of threes in the decimal would repeat infinitely. For suitable precision, the encoder strip would need to be imprinted using a device that provided accuracy to six decimal places, whereas most available devices default to only four or less decimal places of accuracy. As a result of the inherent limitations of the imagesetter to maintain accuracy across the entire length of the film, variations in the distance between adjacent registrations of the encoder strip results. These variations are often manifested as visual xe2x80x9cbandingxe2x80x9d, defects, as illustrated in FIGS. 1 and 2.
The industry has attempted to address this inherent deficiency in several different ways. One method is to use a high resolution imagesetting device to generate a master imprinted on glass (or another permanent material), and using a contact photoprinting process to reproduce encoder strips from that master. This is a relatively slow process, and care must be taken to prevent dust or other contaminants from affecting the contact print. The conventional process of contact printing from a master can lead to loss in image quality, which adversely affects the accuracy or precision of the encoder strip. For wide format encoder strips, the equipment for and corresponding complexity of producing the master can increase the ultimate cost of the encoder strips, and it is necessary to produce a unique master for each version of an encoder strip.
Another method is to imprint markings having only thirty-three thousandths of an inch (0.0033xe2x80x3) width and spacing, rounded down from the corresponding infinite decimal. The result is 150 lines and 150 spaces which extend along a total distance of 0.99xe2x80x3 for each inch of encoder stripxe2x80x94or 99% of the total length of the encoder stripxe2x80x94for a 1% initial error factor overall. The encoder strip is then mounted by stretching the material to its full 100% length and pinning the opposing ends in place.
Another method is to combine a plurality of individually imprinted strips to form a larger encoder strip. One large encoder strip is thus created from adhesively or otherwise secured plurality of smaller strips. Obviously, the resulting process is inefficient and time consuming.
Another method utilized to correct for the inherent limitation in imprinting resolution is to round the line width and spacing upward rather than downward. Using a sixty-seven thousandths inch (0.0067xe2x80x3) combined line width and space rounded up from the corresponding infinite decimal, 150 lines and 150 spaces extend along a total distance of 1.005xe2x80x3 for each inch of encoder stripxe2x80x94or 1.005% of the total length of the encoder stripxe2x80x94for a 0.5% initial error factor overall. While this error is less relative to rounding down (assuming a combined spacing of 0.0067xe2x80x3 can be achieved while maintaining tolerances), the error must either be incorporated into the printed image or corrected in some manner.
One option is to discard a predetermined number of markings and spaces from one of the terminal ends of the encoder strip. For example, in a 46xe2x80x3 wide format, the 0.5% rounding error results in an additional 34.5 lines (45xe2x80x3xc3x97150 lines/in.xc3x970.005) lines. Thirty-four lines and an additional space can be discarded from one terminal end of the encoder strip. Another option is to imprint less than all of the full markings, or a partial line or space (or both) per unit of distance. For example, imprinting 149.5 markings per inch by reducing the width of one line and one space by one half reduces the error to 0.17% (per unit distance or total error). The effect is to build a small error into each unit distance (i.e., 0.5 line width per inch). In either case, either the total image width or discrete rows in the image (or both) will be distorted or incorrect, and the ability to perform such an adjustment is dependent on the tolerances and capabilities of the imprinting apparatus.
Another method is disclosed in U.S. Pat. No. 5,941,649, assigned to Encoder Science Technologies, LLC., assignee of the present invention. That method is practiced by producing a template having the desired number of registration indices at reasonably exact tolerances, but at widths and spacing less than or greater than intended for the registration markings, and therefor having an overall length less than or greater than that of the encoder, and using the template to project an image onto a substrate at a suitable scaling factor to form the encoder having the correct widths and spacing of the registration markings on that substrate.
FIGS. 6 and 7 illustrate one prior art technique for efficiently fabricating a plurality of encoder strips upon a single polymer substrate. An imagesetting device having an internal drum for receiving a polymer substrate, is utilized to imprint a plurality of encoder strips upon the substrate. In the past, the longitudinal axis of each encoder strip has been aligned with the axis of rotation of the drum, AR. This results in the edges of the registration marks being a single vertical line segment, as illustrated in FIG. 4. As described above, the control system and software are limited by cumulative and incremental inaccuracies to control the precise. positioning of the vertical line segments of each registration mark of the encoder strips. As further mentioned, the limitation is manifested as a xe2x80x9cbandingxe2x80x9d error, as illustrated FIG. 1.
The method for fabricating an encoder strip according to this invention produces an encoder strip containing the intended integer number of registration markings (and spaces) per unit distance, over the correct length of that entire encoder strip, so that the registration guide has greater precision, uniformity, and dimensional accuracy. The method overcomes the inherent limitation that an encoder strip composed of lines and spaces disposed in a conventional lines-per-inch (lpi) pattern results in lines and spaces having widths represented as infinite decimals, or finite decimals beyond the available accuracy of equipment used to manufacture those encoder strips.
It is an object of the present invention to provide a method for fabricating a plurality of encoder strips on an optical imagesetting device wherein the edges of each registration mark of each encoder strip are defined by a plurality of disjoint line segments, and not as single line segment as in the prior art. In this regard, the control and software limitations of the imagesetter are removed from the encoder strip by compressing the errors to a non-detectable level. The xe2x80x9cbandingxe2x80x9d error limitation of the imagesetter is still present at a decreased scale, though its effect is not detected by the optical system of a printer device utilizing the improved encoder strip.
Briefly described, the method is practiced by producing an angularly-offset template having the desired number of registration indices at reasonably exact tolerances provided by conventional equipment.